Is it illegal for a defender to hold his hand in front of an offensive players face blocking his vision provided he does not touch the opposing player?

If the defender's hand is near the eyes of the offensive player, then it's a T on the defender.

Always? Without exception?

It's not a T for a post defender that is fronting or beside to deny the entry pass, or vertical plays where the defender is challenging the shot.

I think so, yes. That's the point of the POE this year. Doesn't matter if the player has the ball or not. You hold your hand in front of the eyes of an opponent and it's a T.

I guess you're refering to a "hand in the face" of the shooter. But in this case, you're not really keeping your hand near his/her eyes. You're just putting up a hand briefly and then removing it. Not really posing a danger to the player's eyes.

It is a T if the defender holds his/her hand(s) close to the face and eyes. You can hold a hand up to defend the pass, but you still can't hold your hand directly in front of the opponent's face.

From the NFHS Basketball Guide '04-05 [POE this year]
(page -4-)
The unsporting acts rule was expanded. Purposely obstructing an opponent's vision by waving or placing
hand(s) near his or her eyes now also includes the player with the ball (10-3-7d). Previously the rule only prohibited the act against a player without the ball. Guarding a player's eyes should not be allowed as an effore to obstruct any player's movement and is an unsafe act. It is a technical foul whether or not the player has the ball.
Play 1: A1 throws a pass to A2. B3 hold his or her hand close to and in front of A2's eyes so as to obstruct the vision of A2.
Ruling 1: B3 is whistled for an unsporting technical foul. Team A is awarded two free throws and the ball opposite the scorer's table. The foul counts toward B3's total and toward team B's total.

Play 2: A4 and B5 are running up court in transition and are not directly involved in a play. A4, in an effort to irritate B5, waves his or her hand in B5's face while running up court, just short of making contact but does not.
Ruling 2: A4 is called for an unsporting technical foul. The fact that B5 was not in the act of catching the ball is not relevant. Team B is awarded two free throws and the ball opposite the scorer's table. The foul counts toward A4's total and toward team A's total.

Play 3: A1 has picked up the dribble and is trapped by B3 and B3 near the frontcourt sideline and division line. While playing aggressive defense, B2 "shadows" the ball in an attempt to steal it. Meanwhile, B3 pts both hands near A1's eyes just short of contact and prevents A1 from seeing to make a pass.
Ruling 3: Under the new rule, B3 is called for an unsporting technical foul. While the new rule is not an attempt to curb good defense, shielding a player's eyes is an unsafe and unsporting act, whether or not the player has the ball. Team A is awarded two free throws and the ball opposite the scorer's table. The foul counts toward B3's total and toward team B's total.

Play 4: B5, while playing low-post defense on A4, is positioned on the side A4. To prevent an entry pass into A4, B5 holds a hand up high in font of A4 in order to cut off a passing lane.
Ruling 4: That is a legal play. Even if A4's vision is partially obstructed by B5's hand, B5 is not intentionally blocking A4's vision. The new rule is not intended to apply to plays like fronting pose players or verticality plays.